1. Field of the Invention
The present invention relates to a chip-type piezoelectric-resonator, which is applied to a chip-type filter, oscillator, or discriminator, for example, and a method of manufacturing the same.
2. Description of the Background Art
Conventionally, a piezoelectric-resonator which is applied to a filter, an oscillator or a discriminator is provided with lead wires.
In recent years, however, a surface mounting technique has been employed in order to miniaturize an electronic device, leading to a requirement for a chip-type piezoelectric-resonator having no lead wires. A concrete example of such a chip-type piezoelectric-resonator is now described with reference to FIGS. 11 to 13.
As shown in FIGS. 11 to 13, a piezoelectric-resonator 1 comprises a piezoelectric substrate 2. Split vibrating electrodes 3 and 4 are formed on one major surface of the piezoelectric substrate 2, while still another vibrating electrode 5 is formed on another major surface of the substrate 2 so as to be opposite to the vibrating electrodes 3 and 4. These vibrating electrodes 3, 4 and 5 are respectively connected with terminal electrodes 6, 7 and 8, which are located on an edge of the piezoelectric substrate 2.
This piezoelectric-resonator 1 operates as an energy trapping type double mode piezoelectric-resonator utilizing a thickness vibration mode. The piezoelectric-resonator 1, as described above, has the split vibrating electrodes 3 and 4 and the vibrating electrode 5, which is opposite thereto. As shown in FIG. 12, resin plates 11 and 12 of polyphenylene sulfide (PPS), having cavities 9 and 10 formed in portions corresponding to the vibrating electrodes 3 to 5, respectively, are arranged on both surfaces of the piezoelectric substrate 2, and fixed to the piezoelectric substrate 2 by adhesives 13 and 14. As shown in FIG. 13, external electrodes 15, 16 and 17 are formed on an outer surface of the piezoelectric-resonator 1, and are electrically connected with the terminal electrodes 6, 7 and 8, respectively.
Such a chip-type piezoelectric-resonator 1 is advantageously miniaturized to enable surface mounting.
However, although the resin plates 11 and 12 are bonded to the piezoelectric substrate 2 by the adhesives 13 and 14, the adhesive strength is relatively weak and hence, it is difficult to suppress a spurious response by preventing undesired vibration in a portion other than a vibrating portion of the piezoelectric-resonator 1.
Further, the use of adhesives 13 and 14 may cause permeation of water through interfaces between the piezoelectric substrate 2 and the resin plate 11 and 12, leading to inferior in moisture resistance in the piezoelectric-resonator 1.
In addition, it is difficult to align the vibrating electrodes 3 to 5 with the cavities 9 and 10 of the resin plates 11 and 12 since the resin plates 11 and 12 are bonded to the piezoelectric substrate 2 by the adhesives 13 and 14. This condition hinders automation of the manufacturing process.
The aforementioned cavities that form vibrating spaces for the vibrating electrodes may alternatively be formed by a method of applying a material for forming cavities such as wax, for example, to a region of the piezoelectric substrate that vibrates, and thereafter applying thermosetting protective resin so that the material such as wax migrates into the protective resin when the protective resin is hardened. According to this method, however, it is difficult to attain high accuracy in the areas and the shapes of the portions which are supplied with the cavity forming material. Furthermore, if the material for forming cavities is prepared from wax, the wax which is absorbed in the protective resin may be gasified when external electrodes are formed by sputtering or vacuum deposition under a high temperature condition, to hinder the formation of the electrodes.